Part 3
SHALLOW SEEPAGE
IN THE WEATHERED
BEDROCK CREEP
ZONE

Shallow subsurface seepage tends to percolate most
easily through the weathered rock horizon, shown
here as the reddish colored layer above the parent
greenstone bedrock lying beneath.

The bedrock creep zone lying beneath slopes is
usually dilated and pervious, allowing ephemeral
seepage to percolate through it.


Seepage tends to
flow downslope,
mimicking the
surface
topography
The groundwater
table usually lies
within or close to
the weatherized/
oxidized zone,
with flow
towards the
valley bottom, as
shown here


The bedrock creep
zone is normally
coincident with the
partially saturated
zone lying above the
permanent
groundwater table
Seasonal
fluctuations in
groundwater levels
and flowage usually
occur within this
zone, shown by the
light blue areas
above the dark blue
at lower left

The groundwater table tends to mimic the overlying
topography (except in karst terrain), and is subject to
seasonal and decadal variations, depending on
precipitation and withdrawal
GIS software tends to oversimplify
natural undulations of the ground water
table


Predicted ground water
table (gwt) using kriging
in ArcGIS
Very unrealistic


Predicted gwt using
cokriging with digital
elevation model
Much more realistic
Uncertainty of Reported Groundwater Levels
Measurement
errors very common
Beware of GIS-driven predictions of
groundwater levels



Most geoenvironmental and geohydrology studies presently
employ the geostatistical method of kriging to estimate
groundwater levels, using available well records.
Cokriging that includes dissected ground surface as a
covariable usually provides a more realistic prediction.
Note that multiple regression overestimates in the uplands and
underestimates the true values beneath the flood plains.
Predicted
Groundwater
Elevations

Produced
using cokriging
The most
common
geomorphic
feature in
America

Evolution of a typical
colluvial-filled bedrock
ravine

Bedrock hollows are
excavated during periods of
accelerated erosion, then
slowly infill during periods of
aggradation

Colluvial filled bedrock ravines are the most
common landform in the world. Shallow
groundwater tends to be concentrated towards the
mouths of these zero-order basins


Most colluvial filled ravines
tend to be spoon shaped, as
shown here
Hydraulic uplift promoted by
seepage in the weathered
bedrock tends to
concentrate at bedrock
“highs”, as shown at lower
left

Water levels recorded by piezometers placed on the
bedrock rib adjacent to a colluvial filled ravine (upper
diagram), and another in the axis of the colluvial
deposit (lower diagram). Note how quickly the water
enters the weathered rock and how slowly it drains
from the colluvium
Modeling pore
pressures using
finite elements



Seepage forces tend to
concentrate towards
the lower end of a
spoon-shaped bedrock
ravine, as shown here
This diagram shows
the predicted position
of trapped groundwater
using 2D and 3D
solutions
Two feet of seepage
head can “lift” 4.5 feet
of saturated colluvium
off the slope
Trigger
thresholds for
debris flows


The USGS has
empirically derived
rainfall intensity vs
duration trigger
thresholds for
many areas of the
USA
Those at lower left
are for Central and
Southern California
Coast Ranges
Transient Pore Pressures

An Infinite Slope Analysis can
be performed to illustrate how a
few feet of excess pore pressure
can trigger such failures

Transient pore pressures develop
during periods of sustained
intense precipitation,
occasionally triggering explosive
“blow-out” failures, like that
shown here


Permeability (k) and the hydraulic gradient (i) are more
or less inversely proportional (see sketches above)
Low permeability materials don’t tend to transmit
significant quantities of water